Photovoltaic devices with an interfacial band-gap modifying structure and methods for forming the same

ABSTRACT

A Schottky-barrier-reducing layer is provided between a p-doped semiconductor layer and a transparent conductive material layer of a photovoltaic device. The Schottky-barrier-reducing layer can be a conductive material layer having a work function that is greater than the work function of the transparent conductive material layer. The conductive material layer can be a carbon-material layer such as a carbon nanotube layer or a graphene layer. Alternately, the conductive material layer can be another transparent conductive material layer having a greater work function than the transparent conductive material layer. The reduction of the Schottky barrier reduces the contact resistance across the transparent material layer and the p-doped semiconductor layer, thereby reducing the series resistance and increasing the efficiency of the photovoltaic device.

BACKGROUND

The present disclosure relates to photovoltaic devices, and moreparticularly to photovoltaic devices including an interfacial band-gapmodifying structure and methods of forming the same.

A photovoltaic device is a device that converts the energy of incidentphotons to electromotive force (e.m.f.). Typical photovoltaic devicesinclude solar cells, which are configured to convert the energy in theelectromagnetic radiation from the Sun to electric energy. Each photonhas an energy given by the formula E=hν, in which the energy E is equalto the product of the Plank constant h and the frequency ν of theelectromagnetic radiation associated with the photon.

A photon having energy greater than the electron binding energy of amatter can interact with the matter and free an electron from thematter. While the probability of interaction of each photon with eachatom is probabilistic, a structure can be built with a sufficientthickness to cause interaction of photons with the structure with highprobability. When an electron is knocked off an atom by a photon, theenergy of the photon is converted to electrostatic energy and kineticenergy of the electron, the atom, and/or the crystal lattice includingthe atom. The electron does not need to have sufficient energy to escapethe ionized atom. In the case of a material having a band structure, theelectron can merely make a transition to a different band in order toabsorb the energy from the photon.

The positive charge of the ionized atom can remain localized on theionized atom, or can be shared in the lattice including the atom. Whenthe positive charge is shared by the entire lattice, thereby becoming anon-localized charge, this charge is described as a hole in a valenceband of the lattice including the atom Likewise, the electron can benon-localized and shared by all atoms in the lattice. This situationoccurs in a semiconductor material, and is referred to asphotogeneration of an electron-hole pair. The formation of electron-holepairs and the efficiency of photogeneration depend on the band structureof the irradiated material and the energy of the photon. In case theirradiated material is a semiconductor material, photogeneration occurswhen the energy of a photon exceeds the band gap energy, i.e., theenergy difference of a band gap of the irradiated material.

The direction of travel of charged particles, i.e., the electrons andholes, in an irradiated material is sufficiently random. Thus, in theabsence of any electrical bias, photogeneration of electron-hole pairsmerely results in heating of the irradiated material. However, anexternal field can break the spatial direction of the travel of thecharged particles to harness the electrons and holes formed byphotogeneration.

One exemplary method of providing an electric field is to form a p-i-njunction around the irradiated material. As negative charges accumulatein the p-doped region and positive charges accumulate in the n-dopedregion, an electric field is generated from the direction of the n-dopedregion toward the p-doped region. Electrons generated in the intrinsicregion drift towards the n-doped region due to the electric field, andholes generated in the intrinsic region drift towards the p-dopedregion. Thus, the electron-hole pairs are collected systematically toprovide positive charges at the p-doped region and negative charges atthe n-doped region. The p-i-n junction forms the core of this type ofphotovoltaic device, which provides electromotive force that can powerany device connected to the positive node at the p-doped region and thenegative node at the n-doped region.

BRIEF SUMMARY

A Schottky-barrier-reducing layer is provided between a p-dopedsemiconductor layer and a transparent conductive material layer of aphotovoltaic device. The Schottky-barrier-reducing layer can be aconductive material layer having a work function that is greater thanthe work function of the transparent conductive material layer. Theconductive material layer can be a carbon-material layer such as acarbon nanotube layer or a graphene layer. Alternately, the conductivematerial layer can be another transparent conductive material layerhaving a greater work function than the transparent conductive materiallayer. The reduction of the Schottky barrier reduces the contactresistance across the transparent material layer and the p-dopedsemiconductor layer, thereby reducing the series resistance andincreasing the efficiency of the photovoltaic device.

According to an aspect of the present disclosure, a photovoltaic deviceis provided, which includes a stack of a transparent conductive materiallayer, a Schottky-barrier-reducing layer contacting the transparentconductive material layer, and a p-doped semiconductor layer contactingthe p-doped semiconductor layer. The Schottky barrier across the stackhas a lower contact resistance than a Schottky barrier across anotherstack that includes all layers of the stack less theSchottky-barrier-reducing layer.

According to another aspect of the present disclosure, a method offorming a photovoltaic device is provided. The method includes: forminga transparent conductive material layer on a substrate; forming aSchottky-barrier-reducing layer on the transparent conductive materiallayer; and forming a p-doped semiconductor layer on theSchottky-barrier-reducing layer. The Schottky barrier across a stack ofthe transparent conductive material layer, the Schottky-barrier-reducinglayer, and the p-doped semiconductor layer has less contact resistancethan a Schottky barrier across another stack that includes all layers ofthe stack less the Schottky-barrier-reducing layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a prior art photovoltaicdevice structure.

FIG. 2 is an equivalent circuit for the prior art photovoltaic devicestructure of FIG. 1.

FIG. 3 is a schematic graph of an I-V curve of the prior artphotovoltaic device structure of FIG. 1.

FIG. 4 is a band diagram of a transparent conductive material layer anda p-doped semiconductor layer in the prior art photovoltaic devicestructure of FIG. 1.

FIG. 5 is a graph of an I-V curve for an exemplary prior artphotovoltaic device structure.

FIG. 6 is a vertical cross-sectional view of an exemplary photovoltaicdevice structure according to various embodiments of the presentdisclosure.

FIG. 7 is a band diagram of a junction of a single wall carbon nanotubeand a p-doped silicon material according to the first embodiment of thepresent disclosure.

FIG. 8 is a graph illustrating the resistivity of aluminum-doped zincoxide as a function of weight percentage of aluminum according to asecond embodiment of the present disclosure.

FIG. 9 is a band diagram for first and second exemplary photovoltaicdevice structures according to the first and second embodiments of thepresent disclosure.

FIG. 10A is a vertical cross-sectional view of an exemplary photovoltaicdevice structure after formation of a transparent conductive materiallayer according to embodiments of the present invention.

FIG. 10B is a vertical cross-sectional view of an exemplary photovoltaicdevice structure after formation of a p-doped semiconductor layeraccording to embodiments of the present invention.

FIG. 10C is a vertical cross-sectional view of an exemplary photovoltaicdevice structure after formation of back reflector layers according toembodiments of the present invention.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to photovoltaic devicesincluding an interfacial band-gap modifying structure and methods offorming the same, which are now described in detail with accompanyingfigures. Throughout the drawings, the same reference numerals or lettersare used to designate like or equivalent elements. The drawings are notnecessarily drawn to scale.

As used herein, a crystal structure is “microcrystalline” if the averagegrain size of the material is from 1 nm to 10 microns.

As used herein, a “hydrogenated” semiconductor material is asemiconductor material including incorporated hydrogen therein, whichneutralizes dangling bonds in the semiconductor material and allowscharge carriers to flow more freely.

As used herein, a “semiconductor-material-containing reactant gas”refers to a gas including at least one atom of Si, Ge, or components ofa compound semiconductor material.

As used herein, an element is “optically transparent” if the element istransparent in the visible electromagnetic spectral range having awavelength from 400 nm to 800 nm.

As used herein, a “Schottky-barrier-reducing” element is an element thatis located between two other elements that form a Schottky barrier, inwhich the presence of the Schottky-barrier-reducing element reduces acontact resistance of a structure including the two other elements andthe Schottky-barrier-reducing element relative a structure includingonly the two other elements.

Referring to FIG. 1, a prior art photovoltaic device structure includesa material stack, from top to bottom, of a substrate 110, a transparentconductive material layer 120, a p-doped semiconductor layer 130, anintrinsic semiconductor layer 140, an n-doped semiconductor layer 150, afirst back reflector layer 160, and a second back reflector layer 170.The substrate 110 typically includes an optically transparent material.The transparent conductive material layer 120 functions as a positivenode of the prior art photovoltaic device, and the combination of thesecond back reflector layer 170 functions as a negative node of theprior art photovoltaic device. The first back reflector layer 160 can beoptically transparent, and the combination of the first and second backreflector layers (160, 170) reflect any photons that pass through thestack of the p-doped semiconductor layer 130, the intrinsicsemiconductor layer 140, and the n-doped semiconductor layer 150 toenhance the efficiency of the prior art photovoltaic device.

The p-doped semiconductor layer 130 can include an amorphous p-dopedhydrogenated silicon-containing material or microcrystalline p-dopedhydrogenated silicon-containing material. The amorphous p-dopedhydrogenated silicon-containing material or the microcrystalline p-dopedhydrogenated silicon-containing material can be deposited by flowing asemiconductor-material-containing reactant in hydrogen carrier gas. Inthis case, hydrogen atoms are incorporated in the deposited material ofthe p-doped semiconductor layer 130. The p-doped semiconductor layer 130can include an amorphous p-doped hydrogenated silicon-carbon alloy or amicrocrystalline p-doped hydrogenated silicon-carbon alloy.

Referring to FIG. 2, the functionality of the prior art photovoltaicdevice of FIG. 1 can be approximated by an equivalent circuit thatincludes a current source, a diode, and two resistors. The equivalentcircuit of FIG. 2 approximates a unit area of the prior art photovoltaicdevice of FIG. 1, which provides electrical current that is proportionalto the total irradiated area of the prior art photovoltaic device. Thephotovoltaic current per unit area generated by the prior artphotovoltaic device is referred to as a short-circuit current densityJ_(sc), i.e., the current density generated by the prior artphotovoltaic device if the positive node and the negative node of theprior art photovoltaic device are electrically shorted. Thus, thecurrent source in FIG. 2 generates an electrical current with a currentdensity of the short-circuit current density J_(sc).

Power dissipation through internal leakage current is approximated by ashunt resistance R_(sh). A finite value for the shunt resistance R_(sh)triggers an internal leakage current through the prior art photovoltaicdevice of FIG. 1, and degrades the performance of the prior artphotovoltaic device. The lesser the shunt resistance R_(sh), the greateris the internal power loss due to the internal leakage current.

Power dissipation through internal resistance of the prior artphotovoltaic device of FIG. 1 is approximated by a series resistanceR_(s). A non-zero value for the series resistance R_(s) triggers Jouleloss within the prior art photovoltaic device. The greater the seriesresistance R_(s), the greater is the internal power loss due to theinternal resistance of the prior art photovoltaic device.

Referring back to FIG. 1, a predominant portion of the series resistanceR_(s) is the resistance of a Schottky barrier at the interface betweenthe transparent conductive material layer 120 and the p-dopedsemiconductor layer 130. The Schottky barrier dominates the total valueof the series resistance R_(s) unless significant defects in conductivecomponents, e.g., the transparent conductive material layer 120 or thefirst and second back reflector layers (160, 170), causes the seriesresistance R_(s) to increase abnormally. Thus, in well-functioning priorart photovoltaic devices of FIG. 1, the series resistance R_(s) islimited by the resistance introduced by the Schottky barrier at theinterface between the transparent conductive material layer 120 and thep-doped semiconductor layer 130. In case amorphous hydrogenatedcarbon-containing silicon alloy is employed for the p-dopedsemiconductor layer 130, the series resistance R_(s) of the prior artphotovoltaic device of FIG. 1 is from 20 Ohms-cm² to 30 Ohms-cm². Incase microcrystalline hydrogenated carbon-containing silicon alloy isemployed for the p-doped semiconductor layer 130, the series resistanceR_(s) of the prior art photovoltaic device of FIG. 1 is from 10 Ohms-cm²to 15 Ohms-cm².

The potential difference between the positive node, i.e., the p-dopedsemiconductor layer 130, and the negative node, i.e., the n-dopedsemiconductor layer 150, generates an internal current that flow in theopposite direction to the photocurrent, i.e., the current represented bythe current source having the short-circuit current density J_(sc). Thedark current has the same functional dependence on the voltage acrossthe current source as a diode current. Thus, the dark current isapproximated by a diode that allows a reverse-direction current. Thedensity of the dark current, i.e., the dark current per unit area of theprior art photovoltaic device, is referred to as the dark currentdensity J_(dark). An external load can be attached to an outer node ofthe series resistor and one of the nodes of the current source. In FIG.2, the value the impedance of the load is the value of the actualimpedance of a physical load is divided by the area of the prior artphotovoltaic cell because the equivalent circuit of FIG. 2 describes thefunctionality of a unit area of the prior art photovoltaic cell.

Referring to FIG. 3, a schematic graph of an I-V curve of the prior artphotovoltaic device structure of FIG. 1 is shown. The bias voltage V isthe voltage across the load in the equivalent circuit of FIG. 2. Theopen circuit voltage Voc corresponds to the voltage across the load asthe resistance of the load diverges to infinity, i.e., the voltageacross the current source when the load is disconnected. The inverse ofthe absolute value of the slope of the I-V curve at V=0 and J=J_(sc) isapproximately equal to the value of the shunt resistance R_(sh). Theinverse of the absolute value of the slope of the I-V curve at V=V_(oc)and J=0 is approximately equal to the value of the series resistanceR_(s). The effect of the dark current is shown as an exponentialdecrease in the current density J as a function of the bias voltage Varound a non-zero value of the bias voltage.

The operating range of a photovoltaic device is the portion of the I-Vcurve in the first quadrant, i.e., when both the bias voltage V and thecurrent density J are positive. The power density P, i.e., the densityof power generated from an unit area of the prior art photovoltaicdevice of FIG. 1, is proportional to the product of the voltage V andthe current density J along the I-V curve. The power density P reaches amaximum at a maximum power point of the I-V curve, which has the biasvoltage of V_(m) and the current density of J_(m). The fill factor FF isdefined by the following formula:

$\begin{matrix}{{FF} = {\frac{J_{m} \times V_{m}}{J_{sc} \times V_{oc}}.}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

The fill factor FF defines the degree by which the I-V curve of FIG. 3approximates a rectangle. The fill factor FF is affected by the seriesresistance R_(s) and the shunt resistance R_(th). The smaller the seriesresistance R_(s), the greater the fill factor FF. The greater the shuntresistance R_(sh), the greater the fill factor FF. The theoreticalmaximum for the fill factor is 1.0.

The efficiency η of a photovoltaic device is the ratio of the powerdensity at the maximum power point to the incident light power densityP_(s). In other words, the efficiency η is given by:

$\begin{matrix}{\eta = {\frac{J_{m} \times V_{m}}{P_{s}}.}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

Eq. 2 can be rewritten as:

$\begin{matrix}{\eta = {\frac{J_{sc} \times V_{oc} \times {FF}}{P_{s}}.}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

Thus, the efficiency h of a photovoltaic device is proportional to theshort circuit current density J_(sc), the open circuit voltage V_(oc),and the fill factor FF.

The efficiency η of a photovoltaic device depends on the spectralcomposition of the incident light. For solar cells, the efficiency iscalculated under a standard radiation condition defined as 1 sun, whichemploys the spectrum of the sunlight.

Referring to FIG. 4, a band diagram illustrates the band bending in thep-doped semiconductor layer 130 in the prior art photovoltaic devicestructure of FIG. 1 due to the transparent conductive material layer120. Materials currently available for the transparent conductivematerial layer 120 are n-type materials. A Schottky barrier exits at theinterface between the transparent conductive material layer 120 and thep-doped semiconductor layer 130. The valence band and the conductionband of the p-doped semiconductor layer 130 bend downward at theinterface between the transparent conductive material layer 120 and thep-doped semiconductor layer 130.

In case the transparent conductive material layer 120 is analuminum-doped zinc oxide, the work function of the transparentconductive material layer 120 is about 4.5 eV. In other words, the Fermilevel E_(F) is at 4.5 eV below the vacuum level. Other typical materialsfor the transparent conductive material layer 120 also have a workfunction of about 4.5 eV.

In case the p-doped semiconductor layer 130 includes an amorphoushydrogenated silicon carbon alloy, the band gap of the p-dopedsemiconductor layer 130 is around 1.85 eV. The difference between theFermi level and the valence band of the amorphous hydrogenated siliconcarbon alloy is about 1.0 eV. This is a significant energy barrier, andis the cause of the predominant component of the series resistance R_(s)from 20 Ohms-cm² to 30 Ohms-cm² in the prior art photovoltaic device ofFIG. 1.

Referring to FIG. 5, the significant series resistance R_(s) in theprior art photovoltaic device of FIG. 1 can be manifested as humps in anI-V curve in case the p-doped semiconductor layer 130 includes anamorphous hydrogenated silicon carbon alloy. The portion of the I-Vcurve in the fourth quadrant can be obtained by applying an externalvoltage across the positive and negative terminals of the prior artphotovoltaic device of FIG. 1. The hump in the first quadrant canadversely affect the fill factor FF, and consequently affect theefficiency η adversely.

FIG. 6 is a vertical cross-sectional view of an exemplary photovoltaicdevice structure according to various embodiments of the presentdisclosure. The photovoltaic device structure includes a material stack,from top to bottom, of a substrate 10, a transparent conductive materiallayer 20, a Schottky-barrier-reducing layer 22, a p-doped semiconductorlayer 30, an intrinsic semiconductor layer 40, an n-doped semiconductorlayer 50, a first back reflector layer 60, and a second back reflectorlayer 70.

The substrate 10 is a structure that provides mechanical support to thephotovoltaic structure. The substrate 10 is transparent in the range ofelectromagnetic radiation at which photogeneration of electrons andholes occur within the photovoltaic structure. If the prior artphotovoltaic device is a solar cell, the substrate 10 can be opticallytransparent. The substrate 10 can be a glass substrate. The thickness ofthe substrate 10 can be from 50 microns to 3 mm, although lesser andgreater thicknesses can also be employed.

The transparent conductive material layer 20 includes a material that istransparent in the range of electromagnetic radiation at whichphotogeneration of electrons and holes occur within the photovoltaicdevice structure. If the photovoltaic device structure is employed as asolar cell, the transparent conductive material layer 20 can beoptically transparent. For example, the transparent conductive materiallayer 20 can include a transparent conductive oxide such as afluorine-doped tin oxide (SnO₂:F), an aluminum-doped zinc oxide(ZnO:Al), or indium tin oxide. The thickness of the transparentconductive material layer 20 can be from 300 nm to 3 microns, althoughlesser and greater thicknesses can also be employed.

The Schottky-barrier-reducing layer 22 includes a material that reducesthe Schottky barrier through the stack of the transparent conductivematerial layer 20, the Schottky-barrier-reducing layer 22, and thep-doped semiconductor layer 30 relative a stack (not shown) consistingof a first layer that is the same as the transparent conductive materiallayer 20 and a second layer that is the same as the p-dopedsemiconductor layer 30. Thus, the presence of theSchottky-barrier-reducing layer 22 reduces the Schottky barrier of thestack of the transparent conductive material layer 20, theSchottky-barrier-reducing layer 22, and the p-doped semiconductor layer30 relative to the stack consisting of the first layer and the secondlayer. Correspondingly, the contact resistance through the stack of thetransparent conductive material layer 20, the Schottky-barrier-reducinglayer 22, and the p-doped semiconductor layer 30 is less than thecontact resistance of the stack consisting of the first layer and thesecond layer. In other words, the Schottky barrier across the stack ofthe transparent conductive material layer 20, theSchottky-barrier-reducing layer 22, and the p-doped semiconductor layer30 has lower contact resistance than a Schottky barrier across anotherstack that includes all layers of the stack less theSchottky-barrier-reducing layer 22. The various embodiments of thepresent disclosure differ by the composition of the material in theSchottky-barrier-reducing layer 22.

According to a first embodiment, the Schottky-barrier inducing layer 22is an optically transparent layer including an allotrope of carbon. Inthis embodiment, the exemplary photovoltaic device structure is referredto as a first exemplary photovoltaic device structure. In one case, theSchottky-barrier-reducing layer 22 can be a single wall carbon nanotubelayer. A carbon nanotube is an allotrope of carbon with a cylindricalnanostructure. A single wall carbon nanotube is a carbon nanotube thatdoes not contain any other carbon nanotube therein, and is not containedin another carbon nanotube. Thus, a single wall carbon nanotube is asingle strand of carbon nanotube that stands alone by itself withoutincluding, or being included in, another carbon nanotube. Thecylindrical arrangement of carbon atoms in a single wall carbon nanotubeprovides novel properties that make the carbon nanotube potentiallyuseful in many applications. The diameter of a single wall carbonnanotube is on the order of a few nanometers, while the length of thesingle wall carbon nanotube can be from tens of nanometers to tens ofcentimeters. The chemical bonding of a single wall carbon nanotube iscomposed entirely of sp2 bonds, similar to the bonding of graphite. Thisbonding structure is stronger than the sp3 bonds found in diamonds,providing the single wall carbon nanotube with their unique strength.The single wall carbon nanotube has a work function on the order of 5eV, which is greater than the work function of most transparentconductive material employed for photovoltaic devices.

In another case, the Schottky-barrier-reducing layer 22 can be agraphene layer. Graphene is a structure consisting of carbon as atwo-dimensional sheet. A graphene monolayer has a thickness of about0.34 nm, i.e., which is approximately the atomic diameter of a singlecarbon atom. A graphene layer can exist as a monolayer of atwo-dimensional sheet. Alternately, a graphene layer can exist as astack of a plurality of two-dimensional monolayers of carbon, which donot exceed more than 10 monolayers and is typically limited to less than5 monolayers. Graphene provides excellent in-plane conductivity.Semiconductor devices employing graphene have been suggested in the artto provide high-density and high-switching-speed semiconductor circuits.Carbon atoms are arranged in a two-dimensional honeycomb crystal latticein which each carbon-carbon bond has a length of about 0.142 nm.

According to a second embodiment, the Schottky-barrier-reducing layer 22includes a same material as the transparent conductive material layer20. However, the Schottky-barrier-reducing layer 22 has a differentdoping than the transparent conductive material layer. The difference inthe doping between the transparent conductive material layer 20 and theSchottky-barrier-reducing layer 22 is set such that the presence of theSchottky-barrier-reducing layer 22 reduces the Schottky barrier betweenthe transparent conductive material layer 20 and the p-dopedsemiconductor layer 30. In this embodiment, the exemplary photovoltaicdevice structure is referred to as a second exemplary photovoltaicdevice structure.

In one case, the transparent conductive material layer 20 includes analuminum-doped zinc oxide having an aluminum doping at a first dopantconcentration, and the Schottky-barrier-reducing layer 22 includes analuminum-doped zinc oxide having an aluminum doping at a second dopantconcentration. In this case, the first dopant concentration is greaterthan the second dopant concentration.

In another case, the transparent conductive material layer 20 includes afirst fluorine-doped tin oxide having a fluorine doping at a firstdopant concentration, and the Schottky-barrier-reducing layer 22includes a second fluorine-doped tin oxide having a fluorine doping at asecond dopant concentration. In this case, the first dopantconcentration is greater than the second dopant concentration.

The p-doped semiconductor layer 30 includes an amorphous ormicrocrystalline p-doped semiconductor-containing material. In somecases, the p-doped semiconductor layer 30 can include a hydrogenatedamorphous or microcrystalline p-doped semiconductor-containing material.The presence of hydrogen in the p-doped semiconductor layer 30 canincrease the concentration of free charge carriers, i.e., holes, bydelocalizing the electrical charges that are pinned to defect sites.

A hydrogenated p-doped semiconductor-containing material can bedeposited in a process chamber containing asemiconductor-material-containing reactant gas a carrier gas. Tofacilitate incorporation of hydrogen in the hydrogenated p-dopedsemiconductor-containing material, a carrier gas including hydrogen canbe employed. Hydrogen atoms in the hydrogen gas within the carrier gasare incorporated into the deposited material to form an amorphous ormicrocrystalline hydrogenated p-doped semiconductor-containing materialof the p-doped semiconductor layer 30. The thickness of the p-dopedsemiconductor layer 30 can be from 3 nm to 30 nm, although lesser andgreater thicknesses can also be employed.

The p-doped semiconductor layer 30 can include a silicon-containingmaterial, a germanium-containing material, or a compound semiconductormaterial. In one embodiment, the p-doped semiconductor layer 30 includesa silicon-containing material. The microcrystalline p-doped hydrogenatedsemiconductor-containing material can be a microcrystalline p-dopedhydrogenated silicon-carbon alloy. In this case, a carbon-containing gascan be flown into the processing chamber during deposition of themicrocrystalline p-doped hydrogenated silicon-carbon alloy. The atomicconcentration of carbon in the microcrystalline p-doped hydrogenatedsilicon-carbon alloy of the p-doped semiconductor layer can be from 1%to 90%, and preferably from 10% to 70%. In this case, the band gap ofthe p-doped semiconductor layer 30 can be from 1.7 eV to 2.1 eV.

The intrinsic semiconductor layer 40 includes an intrinsic hydrogenatedsemiconductor-containing material. The intrinsic hydrogenatedsemiconductor-containing material is deposited in a process chambercontaining a semiconductor-material-containing reactant gas a carriergas including hydrogen. Hydrogen atoms in the hydrogen gas within thecarrier gas are incorporated into the deposited material to form theintrinsic hydrogenated semiconductor-containing material of theintrinsic semiconductor layer 40. The intrinsic hydrogenatedsemiconductor-containing material can be amorphous or microcrystalline.Typically, the intrinsic hydrogenated semiconductor-containing materialis amorphous. The thickness of the intrinsic semiconductor layer 40depends on the diffusion length of electrons and holes in the intrinsichydrogenated semiconductor-containing material. Typically, the thicknessof the intrinsic semiconductor layer 40 is from 100 nm to 1 micron,although lesser and greater thicknesses can also be employed.

The intrinsic semiconductor layer 40 can include a silicon-containingmaterial, a germanium-containing material, or a compound semiconductormaterial. In one embodiment, the intrinsic semiconductor layer 40includes a silicon-containing material. The semiconductor material ofthe intrinsic semiconductor layer 40 can be amorphous intrinsic silicon.

The n-doped semiconductor layer 50 includes an n-dopedsemiconductor-containing material. The n-doped semiconductor layer 50can be a hydrogenated material, in which case an n-doped hydrogenatedsemiconductor-containing material is deposited in a process chambercontaining a semiconductor-material-containing reactant gas a carriergas including hydrogen. The n-type dopants in the n-doped semiconductorlayer 50 can be introduced by in-situ doping. Alternately, the n-typedopants in the n-doped semiconductor layer 50 can be introduced bysubsequent introduction of dopants employing any method known in theart. The n-doped semiconductor layer 50 can be amorphous ormicrocrystalline. The thickness of the n-doped semiconductor layer 50can be from 6 nm to 60 nm, although lesser and greater thicknesses canalso be employed.

The n-doped semiconductor layer 50 can include a silicon-containingmaterial, a germanium-containing material, or a compound semiconductormaterial. In one embodiment, the n-doped semiconductor layer 50 includesa silicon-containing material. The semiconductor material of the n-dopedsemiconductor layer 50 can be amorphous n-doped silicon.

The first back reflector layer 60 includes a transparent conductivematerial that is transparent in the range of electromagnetic radiationat which photogeneration of electrons and holes occur within thephotovoltaic device structure. If the photovoltaic device structure isemployed as a solar cell, the first back reflector layer 60 can beoptically transparent. For example, the first back reflector layer 60can include a transparent conductive oxide such as a fluorine-doped tinoxide (SnO₂:F), an aluminum-doped zinc oxide (ZnO:Al), or indium tinoxide. Since such transparent conductive oxide materials are n-typematerials, the contact between the first back reflector layer 60 and then-doped semiconductor layer 50 is Ohmic, and as such, the contactresistance between the first back reflector layer 60 and the n-dopedsemiconductor layer 50 is negligible. The thickness of the backreflector layer 60 can be from 25 nm to 250 nm, although lesser andgreater thicknesses can also be employed.

The second back reflector layer 70 includes a metallic material.Preferably, the metallic material has a high reflectivity in the rangeof electromagnetic radiation at which photogeneration of electrons andholes occur within the photovoltaic device structure. The metallicmaterial can include silver, aluminum, or an alloy thereof. Thethickness of the second back reflector layer 70 can be from 100 nm to 1micron, although lesser and greater thicknesses can also be employed.

Because the resistance due to the Schottky barrier between thetransparent conductive material layer 20 and the p-doped semiconductorlayer 30 is the predominant component of a series resistance in properlyconstructed (i.e., non-defective) photovoltaic devices, the reduction ofthe Schottky barrier results in a dramatic reduction in the seriesresistance in the exemplary photovoltaic device structure according tothe present disclosure compared to prior art photovoltaic devicestructures. For example, while the prior art photovoltaic devicestructure of FIG. 1 has a series resistance from 20 Ohms-cm² to 30Ohms-cm² if an amorphous hydrogenated carbon-containing silicon alloy isemployed for a p-doped semiconductor layer 130, or has a seriesresistance from 10 Ohms-cm² to 15 Ohms-cm² if a microcrystallinehydrogenated carbon-containing silicon alloy is employed for the p-dopedsemiconductor layer 130, photovoltaic device structures according to thepresent disclosure can have a series resistance less than 9 Ohms-cm².Various samples of photovoltaic device structures according to thevarious embodiments of the present disclosure demonstrated a seriesresistance from 4 Ohms-cm² to 9 Ohms-cm².

Referring to FIG. 7, a graph illustrates the J-V characteristics of ajunction of single wall carbon nanotubes and a p-doped silicon materialaccording to the first embodiment of the present disclosure. The J-Vcharacteristics of the junction of the single wall carbon nanotube and ap-doped silicon material approximate the J-V characteristics of thejunction between the Schottky-barrier-reducing layer 22 and the p-dopedsemiconductor layer 30 within the first exemplary photovoltaic devicestructure for the case in which the Schottky-barrier-reducing layer 22includes a layer of single wall carbon nanotubes and the p-dopedsemiconductor layer 30 includes the p-doped silicon material. The J-Vcharacteristics show a diode forward voltage drop of about 0.1V and anestimated internal resistance of about 2 Ohms-cm². The J-Vcharacteristics illustrate a lower contact resistance between theSchottky-barrier-reducing layer 22 and the p-doped semiconductor layer30 than the contact resistance between any transparent conductivematerial layer known in the art and a p-doped semiconductor layer knownin the art.

Referring to FIG. 7, a band diagram of a junction of a single wallcarbon nanotube and a p-doped silicon layer illustrates the mechanismfor the reduction in the contact resistance between theSchottky-barrier-reducing layer 22 and the p-doped semiconductor layer30 according to the first embodiment of the present disclosure. The bandbending Φ_(b) in the p-doped semiconductor layer 30 is the sameirrespective of any selected energy level, i.e., the amount of bandbending the same for the vacuum level, for the valence band E_(v), andfor the conduction band E_(c). The various energy gaps between differentenergy levels in the band diagram are related by the followingequations:

η=E _(F) −E _(V)≈0  (Eq. 1)

Φ_(SWCNT)+η+Φ_(b)−χ_(e) +E _(gSi)  (Eq. 2)

Φ_(SWCNT)=4.97 eV  (Eq. 3)

χ_(e)=4.05 eV  (Eq. 4)

E_(gSi)=1.12 eV  (Eq. 5)

Solving the above equations, the value of band bending Φ_(b) is 0.2 eV,which is substantially smaller than an equivalent band bending of about0.7 eV at an interface between a typical transparent conductive materialand a p-doped silicon material. The smaller band bending at theinterface between Schottky-barrier-reducing layer 22 and the p-dopedsemiconductor layer 30 reduces the Schottky barrier, and consequentlythe accompanying contact resistance, compared with a Schottky barrierbetween a transparent conductive material and a p-doped siliconmaterial.

Single wall carbon nanotubes are transparent. See, for example, Hu etal., “Percolation in Transparent and Conducting Carbon NanotubeNetworks,” Nanoletters, 2004, Vol. 4, No. 12, pp. 2513-2517.

Referring to FIG. 8, a graph illustrates the resistivity ofaluminum-doped zinc oxide as a function of weight percentage of aluminumaccording to a second embodiment of the present disclosure. Theresistivity of aluminum-doped zinc oxide depends on the dopantconcentration of aluminum. A high concentration of aluminum decreasesresistivity of aluminum-doped zinc oxide, and shifts the Fermi levelclose to the conduction band edge. Conversely, a low concentration ofaluminum increases resistivity of aluminum-doped zinc oxide, and shiftsthe Fermi level away from the conduction band toward the mid-gap level,i.e., the energy level midway between the valence band edge and theconduction band edge.

In case aluminum-doped zinc oxide is employed for the transparentconductive material layer 120 in the prior art photovoltaic devicestructure of FIG. 1, the concentration of aluminum in the transparentconductive material layer 120 is constant. A homogeneousheavily-aluminum-doped zinc oxide layer may be employed for thetransparent conductive material layer 120 in the prior art photovoltaicdevice structure of FIG. 1. On the one hand, if a heavily-aluminum-dopedzinc oxide layer contacts a p-doped semiconductor layer 30, the presenceof the Fermi level close to the conduction band edge of theheavily-aluminum-doped zinc oxide layer causes significant the bandbending in the p-doped semiconductor layer 30, and the Schottky barrierbetween the p-doped semiconductor layer 30 and theheavily-aluminum-doped zinc oxide layer. At the same time, the lowresistivity of the heavily-aluminum-doped zinc oxide layer reducesinternal resistance of the photovoltaic device structure that includesthe heavily-aluminum-doped zinc oxide layer. On the other hand, if alightly-aluminum-doped zinc oxide layer contacts a p-doped semiconductorlayer 30, the presence of the Fermi level close to the middle of theband gap of the lightly-aluminum-doped zinc oxide layer reduces the bandbending in the p-doped semiconductor layer 30 relative to the bandbending in the case of a heavily-aluminum-doped zinc oxide layer. TheSchottky barrier between the p-doped semiconductor layer 30 and thelightly-aluminum-doped zinc oxide layer is correspondingly decreased.However, the high resistivity of the lightly-aluminum-doped zinc oxidelayer increases internal resistance of the photovoltaic device structurethat includes the heavily-aluminum-doped zinc oxide layer. The thicknessof the Schottky-barrier-reducing layer 22 can be from 1 nm to 10 nm inthis case. When lightly-aluminum-doped zinc oxide is located at theinterface between p-type semiconductor and heavily-aluminum-doped zincoxide, the lightly-aluminum-doped zinc oxide reduces Schottky barrierheights and heavily-aluminum-doped zinc oxide reduces sheet resistanceof entire zinc oxide stacks.

In one example of the second embodiment, the transparent conductivematerial layer 20 includes an aluminum-doped zinc oxide having analuminum doping at a first dopant concentration, and theSchottky-barrier-reducing layer 22 includes an aluminum-doped zinc oxidehaving an aluminum doping at a second dopant concentration. The firstdopant concentration is greater than the second dopant concentration.The high aluminum concentration of the transparent conductive materiallayer 20 provides low internal resistance in the second exemplaryphotovoltaic device structure. At the same time, the low aluminumconcentration of the Schottky-barrier-reducing layer 22 provides a lowSchottky barrier, and correspondingly, a low contact resistance betweenthe Schottky-barrier-reducing layer 22 and the p-doped semiconductorlayer 30. In one example, the first dopant concentration is selected tobe in the range from 2.0% atomic concentration or greater, and thesecond dopant concentration is selected to be in the range between 0%and 2.0%, although different ranges may be selected for the first andsecond dopant concentrations.

In case fluorine-doped tin oxide is employed for the transparentconductive material layer 120 in the prior art photovoltaic devicestructure of FIG. 1, the concentration of fluorine in the transparentconductive material layer 120 is constant. A homogeneousheavily-fluorine-doped tin oxide layer or a homogeneouslightly-fluorine-doped tin oxide layer may be employed for thetransparent conductive material layer 120 in the prior art photovoltaicdevice structure of FIG. 1. On the one hand, if a heavily-fluorine-dopedtin oxide layer contacts a p-doped semiconductor layer 30, the presenceof the Fermi level close to the conduction band edge of theheavily-fluorine-doped tin oxide layer causes significant the bandbending in the p-doped semiconductor layer 30, and the Schottky barrierbetween the p-doped semiconductor layer 30 and theheavily-fluorine-doped tin oxide layer. At the same time, the lowresistivity of the heavily-fluorine-doped tin oxide layer reducesinternal resistance of the photovoltaic device structure that includesthe heavily-fluorine-doped tin oxide layer. On the other hand, if alightly-fluorine-doped tin oxide layer contacts a p-doped semiconductorlayer 30, the presence of the Fermi level close to the middle of theband gap of the lightly-fluorine-doped tin oxide layer reduces the bandbending in the p-doped semiconductor layer 30 relative to the bandbending in the case of a heavily-fluorine-doped tin oxide layer. TheSchottky barrier between the p-doped semiconductor layer 30 and thelightly-fluorine-doped tin oxide layer is correspondingly decreased.However, the high resistivity of the lightly-fluorine-doped tin oxidelayer increases internal resistance of the photovoltaic device structurethat includes the heavily-fluorine-doped tin oxide layer. The thicknessof the Schottky-barrier-reducing layer 22 can be from 1 nm to 50 in thiscase, although lesser and greater thicknesses can also be employed.

In another example of the second embodiment, the transparent conductivematerial layer 20 includes a fluorine-doped tin oxide having a fluorinedoping at a first dopant concentration, and theSchottky-barrier-reducing layer 22 includes a fluorine-doped tin oxidehaving a fluorine doping at a second dopant concentration. The firstdopant concentration is greater than the second dopant concentration.The high fluorine concentration of the transparent conductive materiallayer 20 provides low internal resistance in the second exemplaryphotovoltaic device structure. At the same time, the low fluorineconcentration of the Schottky-barrier-reducing layer 22 provides a lowSchottky barrier, and correspondingly, a low contact resistance betweenthe Schottky-barrier-reducing layer 22 and the p-doped semiconductorlayer 30. In one example, the first dopant concentration is selected tobe in the range from 2.0% atomic concentration or greater, and thesecond dopant concentration is selected to be in the range between 0%and 2.0%, although different ranges may be selected for the first andsecond dopant concentrations.

In some cases, the Schottky-barrier-reducing layer 22 can include amaterial having a higher resistivity than a material of the transparentconductive material layer 20 if the conductivity of theSchottky-barrier-reducing layer 22 increases with the dopantconcentration in the Schottky-barrier-reducing layer 22, i.e., if theresistivity of the Schottky-barrier-reducing layer 22 decreases with thedopant concentration in the Schottky-barrier-reducing layer 22.

Referring to FIG. 9, a band diagram for first and second exemplaryphotovoltaic devices structure illustrates the mechanism for accordingto the first and second embodiments of the present disclosure. TheSchottly-barrier-reducing layer 22 has a work function that is greaterthan a work function of the transparent conductive material layer by awork function differential Δ. The work function of theSchottly-barrier-reducing layer 22 is lesser than an absolute value ofthe Fermi level E_(F) of the p-doped semiconductor layer 30, i.e., theenergy difference between the vacuum level and the Fermi level energyE_(F) for the p-doped semiconductor layer 30.

In the absence of the Schottky-barrier-reducing layer 22, a directcontact between a transparent conductive material layer and a p-dopedsemiconductor layer causes an energy barrier equivalent to Δ+Φ_(b),i.e., the sum of the difference in the work functions of the transparentconductive material layer 20 and the Schottky-barrier-reducing layer 22and the band bending Φ_(b) at the interface between theSchottky-barrier-reducing layer 22 and the p-doped semiconductor layer30. Because the probability of finding a hole within the valence band ofthe p-doped semiconductor layer is determined by the Fermi-Diracstatistics, the probability of finding a hole in a surface region of thep-doped semiconductor layer 130 at the interface with the transparentconductive material layer 20 decreases almost exponentially withΔ+Φ_(b). Thus, the Schottky barrier and the contact resistance aresignificant. In the present disclosure, the total energy barrier isbroken into two separate barriers, each of which is less significantthan a combined energy barrier. Consequently, the Schottky barrier andthe contact resistance in the combined stack of the transparentconductive material layer 20, the Schottky-barrier-reducing layer 22,and the p-doped semiconductor layer 30 according to the first and secondembodiments of the present disclosure are reduced compared to prior art.

FIG. 10A-10C are sequential vertical cross-sectional views thatillustrate a manufacturing process for forming the exemplaryphotovoltaic device structure of FIG. 6. Referring to FIG. 10A, thesubstrate 10 includes a material that is transparent in the range ofelectromagnetic radiation at which photogeneration of electrons andholes occur within the photovoltaic structure as describe above. Thetransparent conductive material layer 20 is formed on the substrate 10,for example, by deposition.

Referring to FIG. 10B, the Schottky-barrier-reducing layer 22 isdeposited for example, by chemical vapor deposition, evaporation, or anyother known methods of deposition. In case the Schottky-barrier-reducinglayer 22 includes an allotrope of carbon, methods known in the art forforming such an allotrope of carbon can be employed. In case theSchottky-barrier-reducing layer 22 includes a transparent conductivematerial having a different dopant concentration than the transparentconductive material layer 20, methods for forming the transparentconductive material layer 20 can be modified to alter the dopantconcentration in the Schottky-barrier-reducing layer 22.

Referring to FIG. 10C, the p-doped semiconductor layer 30 is depositedin a process chamber containing a semiconductor-material-containingreactant gas and a carrier gas. The p-doped semiconductor layer 30 isformed on the Schottky-barrier-reducing layer 22 in the presence of thesemiconductor-material-containing reactant and the carrier gas in achemical vapor deposition. In case the carrier gas includes hydrogen,the p-doped semiconductor layer 30 includes a hydrogenated p-dopedsemiconductor material. The chemical vapor deposition process can beplasma enhanced chemical vapor process (PECVD) performed at a depositiontemperature from 50° C. to 400° C., and preferably from 100° C. to 350°C., and at a pressure from 0.1 Torr to 10 Torr, and preferably from 0.2Ton to 5 Torr.

The semiconductor-material-containing reactant gas includes at least oneatom of silicon, germanium, or a component semiconductor material of acompound semiconductor material such as GaAs. The p-type dopants in thep-doped semiconductor-containing material of the p-doped semiconductorlayer 30 can be introduced by in-situ doping. Alternately, the p-typedopants in the microcrystalline p-doped hydrogenatedsemiconductor-containing material can be introduced by subsequentintroduction of dopants employing any method known in the art such asplasma doping, ion implantation, and/or outdiffusion from a disposablediffusion source (e.g., borosilicate glass).

The material of the p-doped semiconductor layer 30 can be a p-dopedhydrogenated silicon-carbon alloy. In this case, a carbon-containing gascan be flown into the processing chamber during deposition of thep-doped hydrogenated silicon-carbon alloy.

Subsequently, the intrinsic semiconductor layer 40 is deposited on thep-doped semiconductor layer 30, for example, by plasma-enhanced chemicalvapor deposition. In case the intrinsic semiconductor layer 40 includesan intrinsic hydrogenated semiconductor-containing material, hydrogengas is supplied into the process chamber concurrently with asemiconductor-material-containing reactant gas. The intrinsichydrogenated semiconductor-containing material can be amorphous ormicrocrystalline.

The n-doped semiconductor layer 50 is deposited on the intrinsicsemiconductor layer 40, for example, by plasma-enhanced chemical vapordeposition. In case the n-doped semiconductor layer 50 includes ann-doped hydrogenated semiconductor-containing material, hydrogen gas issupplied into the process chamber concurrently with asemiconductor-material-containing reactant gas. The material of then-doped semiconductor layer 50 can be amorphous or microcrystalline.

The n-type dopants in the n-doped semiconductor layer 50 can beintroduced by in-situ doping. For example, phosphine (PH₃) gas or arsine(AsH₃) gas can be flown into the processing chamber concurrently withthe semiconductor-material-containing reactant gas if the n-dopedsemiconductor layer 50 includes an n-doped silicon-containing materialor an n-doped germanium-containing material. If the n-dopedsemiconductor layer 50 includes an n-doped compound semiconductormaterial, the ratio of the flow rate of the reactant gas for the GroupII or Group III material to the flow rate of the reactant gas for thegroup VI or Group V material can be decreased to induce n-type doping.Alternately, the n-type dopants in the n-doped semiconductor layer 50can be introduced by subsequent introduction of dopants employing anymethod known in the art.

The first back reflector layer 60 is deposited on the n-dopedsemiconductor layer 50 employing methods known in the art. The firstback reflector layer 60 includes a transparent conductive material. Thesecond back reflector layer 70 is subsequently deposited on the firstback reflector layer 70, for example, by electroplating, electrolessplating, physical vapor deposition, chemical vapor deposition, vacuumevaporation, or a combination thereof. The second back reflector layer70 can be a metallic layer.

While the present disclosure has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details can be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A photovoltaic device comprising a stack of a transparent conductivematerial layer, a Schottky-barrier-reducing layer contacting saidtransparent conductive material layer, and a p-doped semiconductor layercontacting said p-doped semiconductor layer, wherein a Schottky barrieracross said stack has a lower contact resistance than a Schottky barrieracross another stack that includes all layers of said stack less saidSchottky-barrier-reducing layer.
 2. The photovoltaic device of claim 1,wherein said Schottky-barrier-reducing layer is an optically transparentlayer including an allotrope of carbon.
 3. The photovoltaic device ofclaim 2, wherein said Schottky-barrier-reducing layer is a single wallcarbon nanotube layer.
 4. The photovoltaic device of claim 2, whereinsaid Schottky-barrier-reducing layer is a graphene layer.
 5. Thephotovoltaic device of claim 1, wherein said Schottky-barrier-reducinglayer includes a same material as said transparent conductive materiallayer and has a different doping than said transparent conductivematerial layer.
 6. The photovoltaic device of claim 5, wherein saidtransparent conductive material layer includes an aluminum-doped zincoxide having an aluminum doping at a first dopant concentration, andsaid Schottky-barrier-reducing layer includes an aluminum-doped zincoxide having an aluminum doping at a second dopant concentration,wherein said first dopant concentration is greater than said seconddopant concentration.
 7. The photovoltaic device of claim 5, whereinsaid transparent conductive material layer includes a firstfluorine-doped tin oxide having a fluorine doping at a first dopantconcentration, and said Schottky-barrier-reducing layer includes asecond fluorine-doped tin oxide having a fluorine doping at a seconddopant concentration, wherein said first dopant concentration is greaterthan said second dopant concentration.
 8. The photovoltaic device ofclaim 5, wherein said Schottky-barrier-reducing layer includes amaterial having a higher resistivity than a material of said transparentconductive material layer.
 9. The photovoltaic device of claim 1,wherein said Schottly-barrier-reducing layer has a work function that isgreater than a work function of said transparent conductive materiallayer and is lesser than an absolute value of a Fermi level of saidp-doped semiconductor layer.
 10. The photovoltaic device of claim 1,wherein a series resistance of said photovoltaic device is equal to orless than 9 Ohms-cm².
 11. The photovoltaic device of claim 1, whereinsaid p-doped semiconductor layer includes a hydrogenated p-dopedsemiconductor-containing material.
 12. The photovoltaic device of claim1, further comprising: an intrinsic semiconductor layer contacting saidp-doped semiconductor layer; and an n-doped semiconductor layercontacting said intrinsic semiconductor layer.
 13. The photovoltaicdevice of claim 12, wherein said intrinsic semiconductor layer includesa hydrogenated amorphous intrinsic semiconductor material.
 14. Thephotovoltaic device of claim 12, wherein said n-doped semiconductorlayer includes hydrogenated n-doped amorphous semiconductor material.15. The photovoltaic device of claim 12, further comprising at least oneback reflector layer located on said n-doped semiconductor layer.
 16. Amethod of forming a photovoltaic device comprising: forming atransparent conductive material layer on a substrate; forming aSchottky-barrier-reducing layer on said transparent conductive materiallayer; and forming a p-doped semiconductor layer on saidSchottky-barrier-reducing layer, wherein a Schottky barrier across astack of said transparent conductive material layer, saidSchottky-barrier-reducing layer, and said p-doped semiconductor layerhas less contact resistance than a Schottky barrier across another stackthat includes all layers of said stack less saidSchottky-barrier-reducing layer.
 17. The method of claim 16, whereinsaid Schottky-barrier-reducing layer is an optically transparent layerincluding an allotrope of carbon.
 18. The method of claim 17, whereinsaid Schottky-barrier-reducing layer is a single wall carbon nanotubelayer.
 19. The method of claim 17, wherein saidSchottky-barrier-reducing layer is a graphene layer.
 20. The method ofclaim 16, wherein said Schottky-barrier-reducing layer includes a samematerial as said transparent conductive material layer and has adifferent doping than said transparent conductive material layer. 21.The method of claim 20, wherein said transparent conductive materiallayer includes an aluminum-doped zinc oxide having an aluminum doping ata first dopant concentration, and said Schottky-barrier-reducing layerincludes an aluminum-doped zinc oxide having an aluminum doping at asecond dopant concentration, wherein said first dopant concentration isgreater than said second dopant concentration.
 22. The method of claim20, wherein said transparent conductive material layer includes a firstfluorine-doped tin oxide having a fluorine doping at a first dopantconcentration, and said Schottky-barrier-reducing layer includes asecond fluorine-doped tin oxide having a fluorine doping at a seconddopant concentration, wherein said first dopant concentration is greaterthan said second dopant concentration.
 23. The method of claim 16,wherein said Schottky-barrier-reducing layer includes a material havinga higher resistivity than a material of said transparent conductivematerial layer.
 24. The method of claim 16, wherein saidSchottly-barrier-reducing layer has a work function that is greater thana work function of said transparent conductive material layer and islesser than an absolute value of a Fermi level energy of said p-dopedsemiconductor layer.
 25. The method of claim 16, wherein a seriesresistance of said photovoltaic device is equal to or less than 9Ohms-cm².